The actor built up to the reveal by tweeting photos of himself, and then himself and co-star David Ramsey, with a green silhouette with a question mark and the words “Mystery Guest,” leading numerous fans on Twitter to joke that the Riddler will make an appearance in Center City. However, Amell ended the mystery this afternoon with a photo of himself sitting on a park bench with Deathstroke.

Deathstroke, aka Slade Wilson, last appeared in the Season 3 episode “The Return.” Played by Manu Bennett, the fan-favorite character was a mentor and ally of Oliver Queen on the island, only to later become his sworn enemy. Oliver seemingly killed Slade by stabbing him the eye, but he survived and returned five years later as the mercenary Deathstroke, who led an assault of Starling City and murdered Moira Queen.

Also starring Emily Bett Rickards, Willa Holland, David Ramsey and Echo Kellum, the fifth season of “Arrow” premiers Oct. 5 at 8 p.m. ET/PT on The CW. The 100th episode is expected to air in December as part of the crossover with “The Flash,” “Supergirl” and “DC’s Legends of Tomorrow.”